Optical multi-beam scanning device and image forming apparatus

ABSTRACT

An optical multi-beam scanning device has a plurality of pre-deflection optical units, an optical path synthesizing member and an excessive light processing member. The excessive light processing member processes excessive light emitted from an excessive light emitting surface with is not an incident surface nor an emitting surface of the optical path synthesizing member. The excessive light processing member has a multi-stage taper constitution with a plurality of taper surfaces having different tilt angles.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation based upon U.S. applicationSer. No. 10/820,750, filed Apr. 9, 2004, incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to an image forming apparatus such as a copyingmachine, a printer, and a composite machine which has a copying functionand a printing function, and an optical multi-beam scanning device whichis mounted on the image forming apparatus. Specifically, the inventionrelates to the optical multi-beam scanning device and the image formingapparatus for a color mode.

Image forming apparatuses mainly adopt a method in which a number oflight beams to be scanned at a time is increased (multi-beam) as speedheightening means of writing optical systems. In writing optical systemsof four-tandem (one pass) color composite machines, it becomesmainstream that a polygon mirror, a scanning lens, and the like areshared in order to reduce the cost.

In the case where the adoption of multi-beam is considered, a method ofsynthesizing laser beams from individual LDs (laser diodes) with oneanother using a beam splitter, a half mirror and the like so as to leadthe synthesized laser beam to a polygon mirror, and a method of leadinga plurality of laser beams to a polygon mirror using an LD array foremitting the laser beams to one direction are considered. From aviewpoint of an installation space, the LD array is advantageous, butwhen a number of scanning light beams increases to 4, 8, . . . ,synthesization using the beam splitter should be used at the same time.In the four-tandem (1 pass) color composite machines, light beams (laserbeams) should be separated by a mirror so as to go to photosensitivedrums for respective color components after deflection. Since separationintervals are too narrow in the method using the LD array, the methodusing the individual LDs is preferable.

There are a lot of optical multi-beam scanning devices which adopt themethod of synthesizing laser beams from the LDs using the beam splitter,the half mirror, and the like so as to lead the synthesized laser beamto the polygon mirror.

Optical parts for switching a ratio of reflection and transmission by adeflecting direction of a beam from the beam splitter, the half mirrorand the like or by the thickness of a reflecting layer, are applied to asynthesizing unit for optical paths of the laser beams. Even in thiscase, a laser beam (excessive light), which moves to a direction (forexample, a reflecting direction) other than an intended direction (forexample, a transmitting direction), is also present, and this laser beambecomes stray light and it might deteriorate the performance of theapparatus.

A technique relating to an optical pickup is disclosed in JapaneseUnexamined Patent Application Publication No. 5-54421. In thispublication, a taper portion of 45° is provided on a portion for housingoptical parts of the optical pickup, and excessive light emitted from apolarized light beam splitter is reflected by the taper portion (anoptical path of the excessive light is folded by 90°) so as to let it goto the outside. There is also described that if the taper portion ispainted black, it is more effective.

Such a stray light countermeasure against the application of the opticalparts such as the beam splitter and the half mirror can be applied onlyto the case where rear positions of stray light emitting surfaces of theoptical parts have a sufficient space. In the optical multi-beamscanning device which leads a plurality of laser beams from a pluralityof LDs to one polygon mirror, since installation positions of therespective optical parts are close to one another, the above methodcannot be applied in most cases from the viewpoint of the space.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an opticalmulti-beam scanning device which is capable of preventing a bad effecton light sources and other optical parts due to stray light of lightbeams from an excessive light emitting surface of an optical pathsynthesizing optical part provided in order to guide a plurality oflight beams to one deflecting surface even when a rear position of theexcessive light emitting surface does not have a sufficient space, andan image forming apparatus which adopts such an optical multi-beamscanning device.

An optical multi-beam scanning device of the present invention includes:a plurality of light sources; pre-deflection optical units for giving apredetermined property to light beams from the light sources, thepre-deflection optical units corresponding to the light sources,respectively; an optical path synthesizing member for aligning opticalpaths of the light beams from all or some of the light sources in ahorizontal scanning direction after the pre-deflection optical unitsgive the predetermined properties to the light beams or while givingthem to the light beams; an excessive light processing member having amulti-stage taper constitution with a plurality of taper surfaces havingdifferent tilt angles for reflecting excessive light emitted from anexcessive light emitting surface which is not an incident surface nor anemitting surface of the optical path synthesizing member; and a lightdeflecting device for deflecting the light beams from the pre-deflectionoptical units corresponding to the light sources to the horizontalscanning direction due to reflection from one surface.

Further, an optical multi-beam scanning device of another aspect if thepresent invention includes: a plurality of light sources; pre-deflectionoptical units for giving a predetermined property to light beams fromthe light sources, the pre-deflection optical units corresponding to thelight sources, respectively; an optical path synthesizing member foraligning optical paths of the light beams from all or some of the lightsources in a horizontal scanning direction after the pre-deflectionoptical units give the predetermined properties to the light beams orwhile giving them to the light beams; an excessive light processingmember having an absorbing surface roughly parallel with an excessivelight emitting surface for absorbing excessive light emitted from theexcessive light emitting surface which is not an incident surface nor anemitting surface of the optical path synthesizing member; and a lightdeflecting device for deflecting the light beams from the pre-deflectionoptical units corresponding to the light sources to the horizontalscanning direction due to reflection from one surface.

The absorbing surface of the excessive light processing member is formedby adhering a light absorbing sheet to the absorbing surface, orrepeating convexo-concave patterns for reflecting and absorbing theexcessive light.

An optical multi-beam scanning device of still another aspect of thepresent invention includes: a plurality of light sources; pre-deflectionoptical units for giving a predetermined property to light beams fromthe light sources, the pre-deflection optical units corresponding to thelight sources, respectively; an optical path synthesizing member foraligning optical paths of the light beams from all or some of the lightsources in a horizontal scanning direction after the pre-deflectionoptical units give the predetermined properties to the light beams orwhile giving them to the light beams; an excessive light processingmember having repeated local patterns for dispersing excessive lightemitted from an excessive light emitting surface which is not anincident surface nor an emitting surface of the optical pathsynthesizing member; and a light deflecting device for deflecting thelight beams from the pre-deflection optical units corresponding to thelight sources to the horizontal scanning direction due to reflectionfrom one surface.

An image forming apparatus of the present invention has an opticalmulti-beam scanning device having a plurality of light sources, acontrol unit for controlling light emitting timing of the light sources,and photoreceptors on which latent images are formed based on lightbeams from the optical multi-beam scanning device. The image formingapparatus adopts the above-mentioned optical multi-beam scanning devicesas the optical multi-beam scanning device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a color image formingapparatus according to a first embodiment of the present invention;

FIG. 2 is a schematic sectional view illustrating components of apost-deflection optical system in an optical multi-beam scanning deviceaccording to the first embodiment;

FIG. 3 is a schematic plan view illustrating components of the opticalmulti-beam scanning device according to the first embodiment;

FIG. 4 is an explanatory diagram illustrating an arrangement of thecomponents of the pre-deflection optical system in the opticalmulti-beam scanning device according to the first embodiment in which anoptical path is a linear optical path;

FIGS. 5A to 5C are diagrams explaining incident light and emitted lightto an optical path synthesizing optical part in the optical multi-beamscanning device according to the first embodiment;

FIG. 6 is a diagram explaining excessive light from the optical pathsynthesizing optical part;

FIG. 7 is a schematic sectional view illustrating an excessive lightprocessing member according to the first embodiment in a verticalscanning direction;

FIG. 8 is a schematic sectional view illustrating the excessive lightprocessing member according to a second embodiment in the verticalscanning direction;

FIG. 9 is a schematic sectional view illustrating the excessive lightprocessing member according to a third embodiment in the verticalscanning direction;

FIGS. 10A and 10B are schematic sectional views illustrating theexcessive light processing member according to a fourth embodiment in ahorizontal scanning direction;

FIGS. 11A and 11B are schematic sectional views corresponding to FIGS.10A and 10B illustrating a modified example of the fourth embodiment;

FIG. 12 is a schematic sectional view illustrating the excessive lightprocessing member according to a fifth embodiment in the horizontalscanning direction;

FIG. 13 is a schematic sectional view illustrating the excessive lightprocessing member according to a sixth embodiment in the horizontalscanning direction;

FIG. 14 is a schematic sectional view in the horizontal scanningdirection illustrating a modified example of the sixth embodiment andcorresponds to FIG. 13; and

FIG. 15 is a schematic sectional view illustrating the excessive lightprocessing member according to a seventh embodiment in the verticalscanning direction.

DETAILED DESCRIPTION OF THE INVENTION

An optical multi-beam scanning device and an image forming apparatusaccording to preferred embodiments of the present invention areexplained below with reference to the drawings.

(A) First Embodiment

FIG. 1 is a diagram illustrating a color image forming apparatus intowhich the optical multi-beam scanning device according to a firstembodiment of the present invention is incorporated. This kind of thecolor image forming apparatus utilizes four sets of various deviceswhich form four kinds of image data separated into color components of Y(yellow), M (magenta), C (cyan), and B (black) and form images for therespective color components corresponding to Y, M, C and B. For thisreason, Y, M, C and B are added to respective reference numerals so thatthe image data for the respective color components and the devices arediscriminated.

As shown in FIG. 1, an image forming apparatus 100 has first to fourthimage forming sections 50Y, 50M, 50C and 50B for forming images forcolor-separated color components.

The image forming sections 50Y, 50M, 50C and 50B are arranged in thisorder below an optical scanning device 1 correspondingly to positionswhere laser beams L (Y, M, C and B) are emitted. The laser beams areused for optically scanning image information about the color componentsusing a first folding mirror 33B and third folding mirrors 37Y, 37M and37C of the optical multi-beam scanning device 1, detailed with referenceto FIGS. 2 and 3.

A conveyance belt 52 for conveying a transfer material on which theimages formed via the image forming sections 50 (Y, M, C and B) aretransferred is arranged below the image forming sections 50 (Y, M, C andB).

The conveyance belt 52 is entrained between a belt driving roller 56 anda tension roller 54 which are rotated to a direction of an arrow by amotor, not shown. The conveyance belt 52 is rotated at a predeterminedspeed to a direction where the belt driving roller 56 rotates.

The image forming sections 50 (Y, M, C and B) are formed into acylindrical shape rotatable to a direction of an arrow, and havephotosensitive drums 58Y, 58M, 58C and 58B, respectively. Electrostaticlatent images corresponding to the images exposed by the opticalscanning device 1 are formed on the photosensitive drums.

Charging devices 60 (Y, M, C and B), developing devices 62 (Y, M, C andB), transfer devices 64 (Y, M, C and B), cleaners 66 (Y, M, C and B),and antistatic devices 68 (Y, M, C and B) are arranged in this order,respectively around the photosensitive drums 58 (Y, M, C and B) alongthe direction where the photosensitive drums 58 (Y, M, C and B) rotate.The charging devices 60 (Y, M, C and B) provide predetermined electricpotentials to the surfaces of the photosensitive drums 58 (Y, M, C andB). The developing devices 62 (Y, M, C and B) supply toner having colorscorresponding to the electrostatic latent images formed on the surfacesof the photosensitive drums 58 (Y, M, C and B) so as to develop theimages. The transfer devices 64 (Y, M, C and B) are arranged so as to beopposed to the photosensitive drums 58 (Y, M, C and B) on a rear surfaceof the conveyance belt 52 with the conveyance belt 52 interveningbetween the transfer devices and the photosensitive drums. The transferdevices 64 (Y, M, C and B) transfer the toner images on thephotosensitive drums 58 (Y, M, C and B) onto a recording medium conveyedby the conveyance belt 52, namely, recording paper P. The cleaners 66(Y, M, C and B) eliminate residual toner on the photosensitive drums 58(Y, M, C and B) which is not transferred when the transfer devices 64(Y, M, C and B) transfer the toner images onto the paper P. Theantistatic devices 68 (Y, M, C and B) remove residual potentials whichremain on the photosensitive drums 58 (Y, M, C and B) after the transferdevices 64 (Y, M, C and B) transfer the toner images.

A paper cassette 70 for housing the recording paper P onto which theimages formed by the image forming sections 50 (Y, M, C and B) aretransferred is arranged below the conveyance belt 52.

A feeding roller 72 which is primarily formed into a half-moon shape andtakes out the paper P housed in the paper cassette 70 one by onestarting from the top is arranged on one end of the paper cassette 70which is close to a tension roller 54.

A register roller 74 for aligning a forward end of one piece of paper Ptaken out of the cassette 70 with a forward end of the toner imageformed on the photosensitive drum 58B of the image forming section 50B(black) is arranged between the feeding roller 72 and the tension roller54.

An absorption roller 76 is arranged in a position which is in a vicinityof the tension roller 54 between the register roller 74 and the firstimage forming section 50Y and is substantially opposed to an outerperiphery of the conveyance belt 52 corresponding to a position wherethe tension roller 54 contacts with the conveyance belt 52. Theadsorption roller 76 provides a predetermined electrostatic adsorptionforce to one piece of paper P conveyed at predetermined timing by theregister roller 74.

Registration sensors 78 and 80 are arranged on the outer periphery ofthe conveyance belt 52 which is at one end of the conveyance belt 52,and in a vicinity of the belt driving roller 56 and substantiallycontacts with the belt driving roller 56 with a predetermined distancein an axial direction of the belt driving roller 56 (since FIG. 1 is afront sectional view, the first sensor 78 positioned on a front side ofthe sheet in FIG. 1 cannot be seen). The registration sensors 78 and 80detect positions of images formed on the conveyance belt 52 or positionsof images transferred onto the paper P.

A conveyance belt cleaner 82 is arranged in a position which is on theouter periphery of the conveyance belt 52 contacting with the beltdriving roller 56 and does not contact with the paper P conveyed by theconveyance belt 52. The conveyance belt cleaner 82 removes toner or slipof paper adhering to the conveyance belt 52.

A fixing device 84 is arranged in a direction where the paper P conveyedby the conveyance belt 52 is separated from the belt driving roller 56and is further conveyed. The fixing device 84 fixes the toner imagestransferred onto the paper P to the paper P.

FIGS. 2 and 3 are diagrams illustrating the optical multi-beam scanningdevice which is incorporated into the image forming apparatus shown inFIG. 1.

The optical multi-beam scanning device 1 has light sources 3Y, 3M, 3Cand 3B, and a light deflecting device 7 as a deflecting unit. The lightsources 3Y, 3M, 3C and 3B output light beams to the first to fourthimage forming sections 50Y, 50M, 50C and 50B shown in FIG. 1,respectively. The light deflecting device 7 deflects (scans) the lightbeams (laser beams) emitted from the light sources 3 (Y, M, C and B)towards imaging surfaces arranged on predetermined positions, namely,outer peripheral surfaces of the photosensitive drums 58Y, 58M, 58C and58B of the first to fourth image forming units 50Y, 50M, 50C and 50Bshown in FIG. 1 at a predetermined linear speed. Predilection opticalsystems 5 (Y, M, C and B) are arranged between the light deflectingdevice 7 and the light sources 3 (Y, M, C and B). A post-deflectionoptical system 9 is arranged between the light deflecting device 7 andthe imaging surfaces.

A direction where the light deflecting device 7 deflects (scans) thelaser beams is called as a horizontal scanning direction. A direction,which crosses perpendicularly to the horizontal scanning direction andan axial line as a basis of a deflecting operation which is performed onthe laser beams by the light deflecting device so that the laser beamsscanned (deflected) by the light deflecting device direct to thehorizontal scanning direction, is called as a vertical scanningdirection.

As shown in FIGS. 3 and 4 (in FIG. 4, arbitrary laser beam L is shown asrepresentative), the pre-deflection optical systems 5 have the lightsources (Y, M, C and B) for respective color components composed ofsemiconductor laser elements, limitary focal lenses 13 (Y, M, C and B),diaphragms 14 (Y, M, C and B), and cylinder lenses 17 (Y, M, C and B),respectively. The limitary focal lenses 13 give predetermined focusingproperties to the laser beams emitted from the light sources 3 (Y, M, Cand B). The diaphragms 14 (Y, M, C and B) give an arbitrary sectionalbeam shape to the laser beams L which pass through the limitary focallenses 13 (Y, M, C and B). The cylinder lenses 17 further give apredetermined focusing property to the laser beams L passing through thediaphragms 14 (Y, M, C and B) in the vertical scanning direction. Thepre-deflection optical systems 5 shape the sectional beam shapes of thelaser beams emitted from the light sources 3 (Y, M, C and B) into apredetermined shape so as to guide the laser beams to the reflectingsurface of the light deflecting device 7.

After an optical path of the laser beam LC of cyan emitted from thecylinder lens 17C is folded by a folding mirror 15C, the laser beam LCtransmits an optical path synthesizing optical part 19 so as to beguided to the reflecting surface of the light deflecting device 7. Afteran optical path of the laser beam LB of black emitted from the cylinderlens 17B is folded by a folding mirror 15B, the laser beam LB isreflected by the optical path synthesizing optical part 19 so as to beguided to the reflecting surface of the light deflecting device 7. Afteran optical path of the laser beam LY of yellow emitted form the cylinderlens 17Y passes through an upper portion of the folding mirror 15C, thelaser beam LY transmits the optical path synthesizing optical part 19 soas to be guided to the reflecting surface of the light deflecting device7. After an optical path of the laser beam LM of magenta emitted fromthe cylinder lens 17M is folded by a folding mirror 15M and the laserbeam LM passes through an upper portion of the folding mirror 15B, thelaser beam LM is reflected by the optical path synthesizing optical part19 so as to be guided to the reflecting surface of the light deflectingdevice 7. In FIG. 4, the folding mirrors 15Y, 15B and 15C and theoptical path synthesizing part 19 are omitted.

FIGS. 5A to 5C are diagrams explaining incident light and emitted lightwith respect to the optical path synthesizing optical part 19. FIG. 5Ais a plan view, FIG. 5B is a diagram (right side view) in which theoptical path synthesizing optical part 19 is viewed from a direction ofan arrow B in FIG. 5A, and FIG. 5C is a diagram (front view) in whichthe optical path synthesizing optical part 19 is viewed from a directionof an arrow C in FIG. 5A.

Heights of positions where the laser beams enter the optical pathsynthesizing optical part 19 become higher in an order of the laser beamLB of black reflected by the optical path synthesizing optical part 19,the laser beam LC of cyan transmitting the optical path synthesizingoptical part 19, the laser beam LM of magenta reflected by the opticalpath synthesizing optical part 19, and the laser beam LY of yellowtransmitting the optical path synthesizing optical part 19.

An excessive light processing member 20, mentioned later, whichcharacterizes the first embodiment is provided in a vicinity of theoptical path synthesizing optical part 19.

The light deflecting device 7 has a polygon mirror 7 a whose, forexample, eight-plane reflecting surfaces (plane reflecting mirror) arearranged into a regular polygon shape and a motor 7 b for rotating thepolygon mirror 7 a at a predetermined speed to the horizontal scanningdirection.

The post-deflection optical system 9 has a pair of imaging lenses 21 (21a and 21 b), a horizontal synchronization optical sensor 23, ahorizontal synchronization folding mirror 25, an optical path correctingelement 27, a plurality of mirrors 33Y, 35Y and 37Y (Yellow), 33M, 35Mand 37M (magenta), 33C, 35C, 37C (Cyan), and 33B (black), and the like.The pair of imaging lenses 21 optimize shapes and positions of the laserbeams L (Y, M, C and B) deflected (scanned) by the polygon mirror 7 a onthe imaging surfaces. The horizontal synchronization optical sensor 23detects the laser beams L in order to conform the horizontalsynchronization of the laser beams L (Y, M, C and B) passing through thepair of imaging lenses 21. The horizontal synchronization folding mirror25 folds the laser beams L towards the horizontal synchronizationoptical sensor 23. The optical path correcting element 27 is arrangedbetween the folding mirror 25 and the horizontal synchronization opticaldetector 23, and makes the laser beams L (Y, M, C and B) for therespective color components reflected towards the horizontalsynchronization optical detector 23 by the folding mirror 25 roughlymatch with the incident position on the detecting surface of thehorizontal synchronization optical detector 23. The mirrors 33Y, 35Y,37Y, 33M, 35M, 37M, 33C, 35C, 37C and 33B guide the laser beams L (Y, M,C and B) for the respective color components emitted from the pair ofimaging lenses 21 to the corresponding photosensitive drums 58 (Y, M, Cand B), respectively.

A beam splitter or a half mirror is adopted as the optical pathsynthesizing optical part 19, but when any one of them is used, as shownin FIG. 6, excessive light is generated. That is to say, incident lightAIN (magenta and black) is reflected by the optical path synthesizingoptical part (for example, beam splitter) 19 so as to be emitted lightAOUT, and a part of the incident light AIN transmits the optical pathsynthesizing optical part 19 so as to become excessive light AR.Incident light BIN (yellow and cyan) transmits the optical pathsynthesizing optical part 19 so as to become emitted light BOUT, and apart of the incident light BIN is reflected by the optical pathsynthesizing optical part 19 so as to become excessive light BR. Boththe excessive light AR and BR is emitted from one same surface of theoptical path synthesizing optical part 19.

The excessive light from the optical path synthesizing optical part 19becomes stray light, and this exerts a bad effect on the light sourcesand the other optical parts. In order to prevent this situation, theexcessive light processing member 20 is provided in a vicinity of theexcessive light emitting surface of optical path synthesizing opticalpart 19.

FIG. 7 is a schematic sectional view of the excessive light processingmember 20 according to the first embodiment in the vertical scanningdirection. In FIG. 7, also the laser beams for color componentsreflected in the optical path synthesizing optical part 19 go straightin order to simplify the explanation.

The excessive light processing member 20 according to the firstembodiment is provided on a rear side of a surface other than theincident surface and the emitting surface of the optical pathsynthesizing optical part (beam splitter) 19 (excessive light emittedsurface). It is composed of a reflecting wall having a multi-stage taperstructure where four taper surfaces 20Y, 20M, 20C and 20B with differentangles are combined.

The taper surface 20B for processing the excessive light (black) whoseemitted position on the optical path synthesizing optical part 19 is thelowest in the vertical scanning direction is set in a position which isthe closest to the optical path synthesizing optical part 19. Further,its tilt angle is set so as to be the smallest. The taper surface 20Cfor processing the excessive light (cyan) whose emitted position on theoptical path synthesizing optical part 19 is the second lowest in thevertical scanning direction is set in a position which is the secondclosest to the optical path synthesizing optical part 19. Further, itstilt angle is set so as to be the second smallest. The taper surface 20Mfor processing the excessive light (magenta) whose emitting position onthe optical path synthesizing optical part 19 is the second highest isset in a position which is the second farthest from the optical pathsynthesizing optical part 19. Further, its tilt angle is set so as to bethe second largest. The taper surface 20Y for processing the excessivelight (yellow) whose emitting position on the optical path synthesizingoptical part 19 is the highest in the vertical scanning direction is setin a position which is the farthest from the optical path synthesizingoptical part 19. Further, its tilt angle is set so as to be the largest.

As shown in FIG. 7, the tilt angles of the processing taper surfaces20Y, 20M, 20C and 20B are set so that reflected return light from thetaper surfaces passes through a position above the optical pathsynthesizing optical part 19 without crossing it.

A broken line shown in FIG. 7 is drawn in order to compare a tapersurface of 45° with the excessive light processing member 20 accordingto the embodiment. Even the tilt angle of the taper surface 20B of theexcessive light processing member 20 with the smallest angle becomeslarger than 45°, and a necessary thickness of the excessive lightprocessing member 20 (a necessary length viewed from a direction wherethe excessive light travels) is thinner than that of the taper surfaceof 45°.

According to the optical multi-beam scanning device and the imageforming apparatus of the first embodiment, even when a sufficient spaceis not provided to the rear position on the excessive light emittingsurface of the optical path synthesizing optical part for guiding aplurality of light beams to one deflecting surface, the light beams formthe excessive light emitting surface is advanced to an unrelateddirection by the taper surfaces including the taper surface with steeptilt angle. For this reason, this can prevent the excessive light frombecoming stray light and exerting a bad effect on the light sources andthe other optical parts.

Since the optical path synthesizing optical part has a multi-stagestructure but simply has the taper surface, the taper shape functions asa drawing angle, and thus it can be formed by injection molding. As aresult, the optical path synthesizing optical part can be provideddirectly on a housing or housing wall of the unit.

(B) Second Embodiment

The optical multi-beam scanning device according to a second embodimentof the present invention and the color image forming apparatus accordingto the second embodiment into which the optical multi-beam scanningdevice is incorporated have the approximately same constitutions asthose in the first embodiment. The constitution of the excessive lightprocessing member 20 provided in the vicinity of the optical pathsynthesizing optical part 19 is slightly different from that in thefirst embodiment.

FIG. 8 is a schematic sectional view illustrating the excessive lightprocessing member 20 according to the second embodiment in the verticalscanning direction, and it corresponds to FIG. 7 in the firstembodiment.

In the first embodiment, the tilt angles of the processing tapersurfaces 20Y, 20M, 20C and 20B of the excessive light processing member20 are set so that the reflected return light from the taper surfacesdoes not cross the optical path synthesizing optical part 19 and passesthrough a position above it. In the second embodiment, however, the tiltangles of the processing taper surfaces 20Y, 20M, 20C and 20B of theexcessive light processing member 20 are set so that the reflectedreturn light from the taper surfaces passes through the optical pathsynthesizing optical part 19 but passes through the upper portion of theoptical part by the side of the light sources from the optical pathsynthesizing optical part 19.

That is to say, in the second embodiment, the tilt angles of theprocessing taper surfaces 20Y, 20M, 20C and 20B are larger than those inthe first embodiment. As a result, a necessary thickness of theexcessive light processing member 20 (a necessary length viewed from thedirection where the excessive light advances) D2 is thinner than thethickness D1 in the first embodiment.

According to the optical multi-beam scanning device and the imageforming apparatus of the second embodiment, the similar effect to thatin the first embodiment can be obtained. When the rear position on theexcessive light emitting surface of the optical path synthesizingoptical part hardly has a sufficient space, the second embodiment ismore suitable than the first embodiment.

(C) Third Embodiment

In a third embodiment of the present invention, the constitution of theexcessive light processing member 20 provided in the vicinity of theoptical path synthesizing part 19 is different from those in the firstand the second embodiments.

FIG. 9 is a schematic sectional view illustrating the excessive lightprocessing member 20 according to the third embodiment in the verticalscanning direction, and corresponds to FIG. 7 in the first embodimentand FIG. 8 in the second embodiment.

The excessive light processing member 20 according to the thirdembodiment partially adopts the technical ideas in the first and thesecond embodiments. That is to say, in the excessive light processingmember 20 according to the third embodiment, similarly to the firstembodiment, the tilt angle of the taper surface 20Y for yellow is set sothat the reflected return light of the incident excessive light is madeto advance above the optical path synthesizing optical part 19 withoutcrossing it. The tilt angles of the other taper surfaces 20M, 20C and20B are, however, set so that the reflected return light from the tapersurfaces passes through the optical path synthesizing optical part 19but passes through the upper portion of the optical part by the side ofthe light sources from the optical path synthesizing optical part 19.

In the third embodiment, a necessary thickness D3 of the excessive lightprocessing member 20 (a necessary length viewed from the direction wherethe excessive light advances) is smaller than the necessary thickness D1in the third embodiment. The thickness D3 is larger than the necessarythickness D2 in the second embodiment.

Also according to the optical multi-beam scanning device and the imageforming apparatus of the third embodiment, the similar effect to that inthe first embodiment can be produced. The third embodiment is suitablefor the case where the rear position on the excessive light emittingsurface of the optical path synthesizing optical part has a space whosesize is in the middle between the suitable spaces in the first andsecond embodiments.

(D) Fourth Embodiment

In a fourth embodiment of the present invention, the constitution of theexcessive light processing member 20 provided in the vicinity of theoptical path synthesizing optical part 19 is different from those in theabove embodiments.

FIGS. 10A and 10B are schematic sectional views illustrating theexcessive light processing member 20 according to the fourth embodimentin the horizontal scanning direction (correspond to the plan view ofFIG. 6).

In the excessive light processing member 20 according to the fourthembodiment, a surface opposed to the excessive light emitting surface ofthe optical path synthesizing optical part 19 has a sawtooth shape inthe cross section in the horizontal scanning direction. The sawtoothshape is composed by repetition of surfaces parallel with the advancingdirection of the excessive light and surfaces tilted by 45° to theadvancing direction of the excessive light. The excessive light from theoptical path synthesizing optical part 19 is reflected three times inthe excessive light processing member 20 of the fourth embodiment and isreturned to the optical path synthesizing optical part 19 as shown in apartial enlarged diagram of FIG. 10B.

The excessive light itself from the optical path synthesizing opticalpart 19 has weak power, and is reflected by the excessive lightprocessing member 20 three times so as to be returned to the opticalpath synthesizing optical part 19. For this reason, due to absorptionwith predetermined absorptance at every time of reflection, the returnlight to the optical path synthesizing optical part 19 has power whichdoes not exert a bad effect on the light sources and the other opticalparts.

Further, the excessive light processing member 20 has the cross sectionwith the sawtooth shape but can be an approximately plate-shaped member,or can be formed as one surface of the housing so as to be capable ofbeing provided very closely to the excessive light emitting surface ofthe optical path synthesizing optical part 19.

According to the optical multi-beam scanning device and the imageforming apparatus of the fourth embodiment, even when the rear positionon the excessive light emitting surface of the optical path synthesizingoptical part for guiding a plurality of light beams to one deflectingsurface does not have an enough space, the light beams from theexcessive light emitting surface are multiply reflected by the excessivelight processing member so as to be absorbed at every time ofreflection. This can prevent the excessive light from becoming straylight and exerting a bad effect on the light sources and the otheroptical parts.

FIGS. 11A and 11B are diagrams illustrating a modified example of thefourth embodiment, and correspond to FIGS. 10A and 10B. In the excessivelight processing member 20 shown in FIGS. 11A and 11B, the sawtoothshape has an acute angle so that a number of reflection times increasesfor the absorption of the excessive light.

The modified example of the fourth embodiment is such that the repeatingdirection of the sawtooth shape is not the directions shown in FIGS.10A, 10B, 11A and 11B. For example, the direction may be the verticalscanning direction.

(E) Fifth Embodiment

In a fifth embodiment of the present invention, the constitution of theexcessive light processing member 20 provided in the vicinity of theoptical path synthesizing optical part 19 is different from those in theabove embodiments.

FIG. 12 is a schematic sectional view illustrating the excessive lightprocessing member 20 according to the fifth embodiment in the horizontalscanning direction.

The excessive light processing member 20 according to the fifthembodiment is structured as a plate-shaped member so as to have asurface opposed to the excessive light emitting surface of the opticalpath synthesizing optical part 19. In another manner, the excessivelight processing member 20 is structured as one surface of the housingor the like, and a light absorbing sheet 20S is provided to the surfaceopposed to the excessive light emitting surface of the optical pathsynthesizing optical part 19.

Also according to the optical multi-beam scanning device and the imageforming apparatus according to the fifth embodiment, the light absorbingsheet 20S absorbs the excessive light from the optical path synthesizingoptical part. For this reason, the same effect as that in the fourthembodiment can be produced.

(F) Sixth Embodiment

In a sixth embodiment of the present invention, the constitution of theexcessive light processing member 20 provided in the vicinity of theoptical path synthesizing optical part 19 is different from those in theabove embodiments.

FIG. 13 is a schematic sectional diagram illustrating the excessivelight processing member 20 according to the sixth embodiment in thehorizontal scanning direction.

The excessive light processing member 20 according to the sixthembodiment is structured as, for example, a plate-shaped member. Aplurality of small local reflecting surfaces with a hemispherical shape(or a semicylindrical shape which extends only to a certain directionsuch as the vertical scanning direction) are arranged on the surfaceopposed to the excessive light emitting surface of the optical pathsynthesizing optical part 19, for example, in every direction (forexample, the horizontal scanning direction and the vertical scanningdirection). The excessive light is reflected from the local reflectingsurfaces so as to be dispersed to various directions. As a result, thepower of the reflected light per unit area becomes very weak.

The sixth embodiment, therefore, can prevent the excessive light frombecoming stray light and exerting a bad effect on the light sources andthe other optical parts.

FIG. 14 is a diagram illustrating a modified example of the sixthembodiment, and corresponds to FIG. 13. The excessive light processingmember 20 shown in FIG. 14 has a shape such that the entire surfaceincluding the small local reflecting surfaces with the hemispherical orsemicylindrical shape is along a part of the spherical or cylindricalsurface. This minimizes an amount of the dispersed light reflected fromthe local reflecting surfaces which returns to the excessive lightemitting surface of the optical path synthesizing optical part 19.

(G) Seventh Embodiment

In a seventh embodiment of the present invention, the constitution ofthe excessive light processing member 20 provided in the vicinity of theoptical path synthesizing optical part 19 is different from those in theabove embodiments.

FIG. 15 is a schematic sectional view illustrating the excessive lightprocessing member 20 according to the seventh embodiment in the verticalscanning direction.

The seventh embodiment can be applied to the case where the optical pathsynthesizing optical part 19 receives a pressing force of a plate springmember 150 so that its position is regulated by a stopper section 101,and the excessive light emitting surface of the optical pathsynthesizing optical part 19 receives the pressing force of the platespring member 150. One surface of the plate spring member 150 opposed tothe excessive light emitting surface of the optical path synthesizingoptical part 19 has a function of the excessive light processing member20 according to any one of the embodiments. In an example shown in FIG.15, one surface of the plate spring member 150 has the function of theexcessive light processing member 20 according to the fourth embodiment(specifically, the modified example of the fourth embodiment).

According to the seventh embodiment, new surface finishing or the likeis necessary for the plate spring member 150, but the excessive lightcan be prevented from becoming stray light and exerting a bad effect onthe light sources and the other optical parts without providing a newmember.

(H) Another Embodiment

The above embodiments explain various modified examples, but a modifiedexample explained below can be given.

The first to the third embodiments explain that the taper surfaces withdifferent tilt angles whose number is the same as that of the four kindsof the laser beams are provided. The number of the taper surfaces withdifferent angles is not, however, limited to a number of the laser beamsas long as their combination can make the thickness of the excessivelight processing member 20 thin. The number may be, therefore, two ormore. Further, the excessive light processing member 20 may have acurved surface where the tilt angle continuously changes (for example, asurface having a parabola-shaped cross section). In this case,reflection (diffuse reflection) to an unintended direction in a fixedswitching position of the tilt angle can be suppressed.

The first to the third embodiments explain that the reflected light fromthe excessive light processing member 20 directs to a position higherthan the incident light viewed from the vertical scanning direction. Thetilt angles of the taper surfaces may be selected so that the reflectedlight from the excessive light processing member 20 directs downwardviewed from the vertical scanning direction. In another manner, positiveand negative tilt angles are present so that the light are reflectedupward viewed from the vertical scanning direction by a certain tapersurface and reflected downward viewed form the vertical scanningdirection by another taper surface.

The first to the third embodiments explain that the excessive light isallowed to go upward or downward with respect to the vertical scanningdirection, but may be allowed to go to the horizontal scanningdirection.

The fourth embodiment explains that the reflection at a plurality oftimes for absorption of the excessive light is executed by utilizing thesawtooth shape. The reflection at a plurality of times may be executedby utilizing another repeated shape such as a triangular wave shape.

The sixth embodiment explains that the dispersed reflection is executedby the repetition of the local reflecting surfaces with thehemispherical or semicylindrical shape. The dispersed reflection(diffuse reflection) may be executed by rough finishing of the surfaceand the like.

The above embodiments explain the optical multi-beam scanning device forallowing the four beams to enter one surface of the polygon mirror. Thepresent invention can be, however, applied to optical multi-beamscanning devices for allowing a plurality of beams such as two beams,seven beams or eight beams enter one surface of the polygon mirror. Thatis to say, the present invention can be applied to optical multi-beamscanning devices having one or more optical path synthesizing opticalparts (beam splitter or half mirror) for synthesizing optical paths ofbeams. Further, the present invention is not limited to the color mode,and can be applied to optical multi-beam scanning devices for monochromemode having one or more optical path synthesizing parts (beam splitteror half mirror) for synthesizing optical paths of beams.

The position of the pre-deflection optical system in the optical pathsynthesizing optical part is not limited to the position explained inthe embodiments. A cylinder lens for giving a predetermined focusingproperty in the vertical scanning direction, for example, may beprovided on a lower stream side of the optical path synthesizing opticalpart.

1. An optical multi-beam scanning device, comprising: a plurality oflight sources; pre-deflection optical units for giving a predeterminedproperty to light beams from the light sources, the pre-deflectionoptical units corresponding to the light sources, respectively; anoptical path synthesizing member for aligning optical paths of the lightbeams from all or some of the light sources in a horizontal scanningdirection after the pre-deflection optical units give the predeterminedproperties to the light beams or while giving them to the light beams;an excessive light processing member having a multi-stage taperconstitution with a plurality of taper surfaces having different tiltangles for reflecting excessive light emitted from an excessive lightemitting surface which is not an incident surface nor an emittingsurface of the optical path synthesizing member; and a light deflectingdevice for deflecting the light beams from the pre-deflection opticalunits corresponding to the light sources to the horizontal scanningdirection due to reflection from one surface.
 2. The optical multi-beamscanning device according to claim 1, wherein the excessive lightprocessing member has the taper surfaces with different angles whosenumber is the same as that of the optical paths.
 3. The opticalmulti-beam scanning device according to claim 1, wherein the tilt anglesof all the taper surfaces in the excessive light processing member areset so that reflected light from the taper surfaces does not enter theexcessive light emitting surface of the optical path synthesizingmember.